Hole in the front of the cylinder head.

No, sir. The one and only air injection port in air injection-equipped '72-up Slant-6 heads is at the rear, on the manifold mounting face, aft of the rearmost exhaust port.

'84 FPC shows a cup plug for the front hole, part № 5203 340. Crosses to a Dorman № 555067, specced as a 39/64" cup plug.
Thank you, I'll stop by the auto parts store when I get a chance and let you know if it works.